This invention relates to the field of optical systems, and more particularly to methods and apparatus for monitoring the power of a multi-wavelength optical signal.
Various forms of optoelectronic devices have been developed and have found widespread use including, for example, semiconductor lasers, semiconductor photodiodes, semiconductor photo detectors, etc. For some of these applications, an optoelectronic emitter such as a semiconductor laser is coupled to an optoelectronic detector (e.g., photodiode or Resonant Cavity Photo Detector) through a fiber optic link or even free space. This configuration can provide a high-speed communication path, which, for many applications, can be extremely beneficial.
The increased use of all-optical fiber networks as backbones for global communication systems has been based in large part on the extremely wide optical transmission bandwidth provided by optical fiber. This has led to an increased demand for the practical utilization of the optical fiber bandwidth, which can provide, for example, increase communication system user capacity. In the prevailing manner for exploiting optical fiber bandwidth, wavelength-division multiplexing (WDM) and wavelength-division demultiplexing (WDD) techniques are used to enable the simultaneous transmission of multiple independent optical data streams, each at a distinct wavelength, on a single optical fiber, with wavelength-selective WDM and WDD control provided for coupling of the multiple data streams with the optical fiber on a wavelength-specific basis. With this capability, a single optical fiber can be configured to simultaneously transmit several optical data streams, e.g., ten optical data streams, that each might not exceed, say, 10 Gb/s, but that together represent an aggregate optical fiber transmission bandwidth of more than, say, 100 Gb/s.
In order to increase the aggregate transmission bandwidth of an optical fiber, it is generally preferred that the wavelength spacing of simultaneously transmitted optical data streams, or optical data xe2x80x9cchannels,xe2x80x9d be closely packed to accommodate a larger number of channels. In other words, the difference in wavelength between two adjacent channels is preferably minimized.
In addition, in WDM communications systems as well as in many other applications, it is often desirable to monitor the power of each data channel. The power of each data channel may vary for a variety of reasons including, for example, changing operating conditions such as operating voltage, operating temperature, device degradation, etc. If the power of one or more of the data channels falls outside of a desired range, the reliability of the communications link can significantly degrade. In some systems, it is possible to provide a separate detector for each data channel. However, this is not always possible, and in many cases, can add significant cost to the system.
The present invention provides methods and apparatus for monitoring the power level of a multi-wavelength optical signal. Also provided are methods and apparatus for adjusting the power level of selected optical emitters to compensate for the changes in power levels.
In one illustrative embodiment of the present invention, a detector is used to detect two or more wavelengths of light, and to provide an indication of the power level of each wavelength of light in a multi-wavelength optical signal. The detector may include, for example, a first absorbing layer, a second absorbing layer situated below the first absorbing layer, and an intermediate layer situated between the first absorbing layer and the second absorbing layer. In some embodiments, the first absorbing layer and the second absorbing layer are a first conductivity type, and the intermediate layer is a second conductivity type. In this configuration, a first PN junction may be formed between the first absorbing layer and the intermediate layer, and a second PN junction may be formed between the second absorbing layer and the intermediate layer.
The detector may receive a multi-wavelength optical signal. The multi-wavelength optical signal may be provided by, for example, two or more optoelectronic emitters, such as semiconductor lasers, semiconductor light emitting diodes, etc., each providing a different wavelength of light. The first absorbing layer may absorb a first portion of a first wavelength of light and a second portion of a second wavelength of light. For example, the first absorbing layer may absorb a majority of the first wavelength of light and a minority of the second wavelength of light. The second absorbing layer, which is preferably situated below the first absorbing layer, may absorb a third portion of the first wavelength of light and a fourth portion of the second wavelength of light. For example, the second absorbing layer may absorb a minority of the first wavelength of light and a majority of the second wavelength of light. The relative portions of light absorbed by the first absorbing layer and the second absorbing layer may be controlled by, for example, the materials and/or thickness used for the first absorbing layer and/or second absorbing layer. In a preferred embodiment, the first absorbing layer and the second absorbing layer are adapted to collectively absorb all or substantially all of the first wavelength of light and the second wavelength of light.
When the power of either the first wavelength of light or the second wavelength of light changes, the relative portions absorbed by the first absorbing layer and the second absorbing layer may also change. For example, if the power level of the first wavelength of light decreases by ten percent, the overall light absorbed by the first absorbing layer may decrease more than the overall light absorbed by the second absorbing layer. In this example, this is because the first absorbing layer absorbs more of the first wavelength of light than the second absorbing layer. Thus, by using a measure of the light absorption in the first absorbing layer and a measure of the light absorption in the second absorbing layer, an indication of the change in the power level of the first wavelength of light and/or the second wavelength of light can be identified.
In some embodiments, a ratio of the measure of the light absorption in the first absorbing layer and the second absorbing layer is used to identify which wavelength of light experienced a power level change. In some embodiments, a sum of the measure of the light absorption in the first absorbing layer and the second absorbing layer may further be used to identify which wavelength of light experienced a power change, and/or if more than one wavelength of light experienced a power change. While only two wavelengths of light are used in this example, it is contemplated that any number of wavelengths may be used.
In another illustrative embodiment of the present invention, an optical transmitter may be provided that includes a first and second electrical input signal. A first modulator may modulate the first electrical input signal with a first electrical power monitor signal to produce a first electrical modulated signal. The first electrical modulated signal may be provided to a corresponding optoelectronic emitter to produce a first optical output signal. The first electrical power monitor signal may operate at a frequency that is substantially less than the frequency or data rate of the first electrical input signal so that the first electrical power monitor signal represents an average power output of the corresponding optoelectronic emitter. In some embodiments, the first modulator may xe2x80x9camplitudexe2x80x9d modulate the first electrical input signal with the first electrical power monitor signal, with the amplitude of the first electrical power monitor signal substantially less than the amplitude of the first electrical input signal.
A second modulator may also be provided for modulating the second electrical input signal with a second electrical power monitor signal to produce a second electrical modulated signal. The second electrical modulated signal may be provided to an optoelectronic emitter to produce a second optical output signal. The second electrical power monitor signal may operate at a frequency that is substantially less than the frequency or data rate of the second electrical input signal so that the second electrical power monitor signal represents an average power output of the corresponding optoelectronic emitter. In some embodiments, the second modulator may xe2x80x9camplitudexe2x80x9d modulate the second electrical input signal with the second electrical power monitor signal, with the amplitude of the second electrical power monitor signal substantially less than the amplitude of the second electrical input signal.
An optical combiner may combine the first optical output signal and the second optical output signal into a common optical output signal. A detector may then be used to monitor the common optical output signal, and produce a corresponding electrical detection signal. In one embodiment, the detector is a wide band detector.
A filter or the like may be used to frequency separate the first power monitor signal and the second power monitor signal from the electrical detection signal, resulting in a first detected power monitor signal and a second detected power monitor signal. The power of the first optoelectronic emitter and the second optoelectronic emitter may then be adjusted based on one or more characteristics of the first detected power monitor signal and the second detected power monitor signal. For example, the power of the first optoelectronic emitter and the second optoelectronic emitter may be adjusted based on the amplitude of the first detected power monitor signal and the amplitude of the second detected power monitor signal. While only two wavelengths are used in this example, it is contemplated that any number of wavelengths may be used.
Rather than using a broad band detector, it is contemplated that the optical transmitter may include a detector that can help provide an indication of the power level of selected wavelengths of light. For example, if four electrical input signals are provided, two of the electrical input signals may be modulated with a first electrical power monitor signal and the remaining two electrical input signals may be modulated with a second electrical power monitor signal. The four modulated electrical input signals may then be provided to four corresponding optoelectronic emitters to produce four optical output signals. An optical combiner may be used to combine the four optical output signals into a common optical output beam.
The detector may include a first absorbing layer, a second absorbing layer situated below the first absorbing layer, and an intermediate layer situated between the first absorbing layer and the second absorbing layer. The first absorbing layer may absorb a different proportion of the each of the four optical output signals, and the second absorbing layer may absorb the remaining portion of each of the four optical output signals. When the power of any of the four optical output signals changes, the relative portions absorbed by the first absorbing layer and the second absorbing layer may also change. For example, if the power level of a first wavelength of light decreases by ten percent, the overall light absorbed by the first absorbing layer may decrease more than the overall light absorbed by the second absorbing layer, particularly if the first absorbing layer absorbs more of the first wavelength of light.
In one illustrative embodiment, a first electrical input signal and a third electrical input signal are modulated with a first electrical power monitor signal to produce a first electrical modulated signal and a third electrical modulated signal. Likewise, a second electrical input signal and a fourth electrical input signal are modulated with a second electrical power monitor signal to produce a second electrical modulated signal and a fourth electrical modulated signal. The first, second, third and fourth electrical modulated signals are provided to corresponding optoelectronic emitters to produce first, second, third and fourth optical output signals.
A detector having a first absorbing layer and a second absorbing layer receives the first, second, third and fourth optical output signals. The first absorbing layer may absorb a different proportion of the each of the four optical output signals, and the second absorbing layer may absorb substantially the remaining portion of each of the four optical output signals. Using a measure of the light absorption in the first absorbing layer and the second absorbing layer, an indication of change in the power level of the first/fourth optical output signal pair, or the second/third optical output signal pair can be identified.
A filter or the like can be used to separate out the first power monitor signal from the first optical signal and the third optical signal, and the second power monitor signal from the second optical signal and the fourth optical signal. The power of the first optoelectronic emitter may then be adjusted if it is determined that the first optical signal/fourth optical signal pair had an increase or decrease in power level and said first power monitor signal indicates that the first optical signal or the third optical signal had an increase or decrease in power level. Likewise, the power of the second optoelectronic emitter may be adjusted if it is determined that the second optical signal/third optical signal pair had an increase or decrease in power level and the second power monitor signal indicates that said second optical signal or fourth optical signal had an increase or decrease in power level. The power of the third optoelectronic emitter may be adjusted if it is determined that the second optical signal/third optical signal pair had an increase or decrease in power level and said first power monitor signal indicates that the first optical signal or third optical signal had an increase or decrease in power level. Finally, the power of the fourth optoelectronic emitter may be adjusted if it is determined that the first optical signal/fourth optical signal pair had an increase or decrease in power level and said second power monitor signal indicates that said second optical signal or fourth optical signal had an increase or decrease in power level.